The present invention relates to sodium percarbonate granules, a process for the production of such granules, and their use in washing powders.
Hitherto, many processers have been proposed for the production of sodium percarbonate granules, but the majority of such processes produce granules which have an undesirably low resistance to attrition, or have relatively poor stability when stored either by themselves, or mixed with other constituents of washing powders.
One interesting process for the production of sodium percarbonate granules enables certain of the aforementioned difficulties to be mitigated. The process comprises the steps of introducing a solution of hydrogen peroxide and a solution of sodium carbonate into a fluidized bed containing seeds of smaller size than the granules to be obtained, and is described in French patent application No. 70.01315, filed on Jan. 14, 1970 in the name of Solvay et Cie, and published under No. 2,076,430. The granular product of this process in general has a satisfactory resistance to abrasion, but, in practice, operation of the process can present some difficulties. In fact, if each solution is sprayed into the fluidized bed using a separate atomizer, it is difficult to produce a sufficiently intimate mixture of the two solutions within the fluidized bed to give granules having the desired homogeneity. However, if the two solutions are introduced into the fluidized bed simultaneously by the same atomizer, there is a substantially increased tendency for premature crystallization to take place in the atomizer, leading to blockages of it, which therefore causes frequent stoppages of the plant.
In order to circumvent the disadvantages of introducing solutions directly into the fluidized bed, Solvay et Cie have proposed in French patent application No. 71.40860, filed on Nov. 15, 1971 and published under No. 2,160,251, a process in which the seeds are impregnated with the two solutions in a separate stage, the impregnated seeds are then passed into a fluidized bed dryer, and all particles which are smaller than the required particle size are collected at the outlet from the fluidized bed and recycled to the impregnation stage. Although this process produces homogeneous granules, it suffers from the disadvantage that a very large proportion of the granules is recycled and only a small proportion of the granules retained as product, so that the productivity of the apparatus is thus lower than desired.
Moreover, the sodium percarbonate granules obtained by the previously disclosed processes are not sufficiently stable if they are contaminated with certain metal impurities, such as iron.